The human trematode parasite, Schistosoma mansoni, causes a disease of major health importance in many regions of the world. During the life cyle of this organism, penetration of host tissues occurs at almost every stage. The egg stage of this organism probably elaborates enzymes to effect its passage through its vertebrate host tissues. Various antigens, one or more of which may be these enzymes, appear to elicit the host's immune response which is the principal cause of pathology of schistosomiasis. Penetration of the miracidium into the snail host is also likely to be mediated by lytic enzymes. Proteolytic enzymes from eggs and miracidia will be isolated, characterized and compared with the proteases isolated from cercariae of the same organism. Comparison will be based on: 1) physical characteristics (molecular weight, sedimentation coefficient, amino acid composition); 2) enzymatic techniques (substrate preferences with connective tissue components and susceptibility to inhibitors); and 3) immunological cross reactivity.